In the past, there have been recording media on which records are made thermally by the use of light beams such as lasers having a high energy density. In such thermal recording media, information is recorded by the difference in optical density on the recording layer. The recording layer has a high optical density and absorbs light beams of high energy density which impinge thereon. The absorption of light brings about a local temperature rise, causing a thermal change such as melting-cohesion and evaporation to take place in the recording layer. The information thus recorded can be read by irradiating the laser beam for reading (cf. U.S. Pat. Nos. 4,188,214, 4,291,119, 4,216,501, and 4,233,626.
These types of thermal recording medium usually do not need development and fixing processes and darkroom operations because of their insensitivity to room light. In addition, they provide high-contrast images and permit information to be added later on.
Recording on such a thermal recording medium is usually accomplished by converting the information to be recorded into electrical time series signals and scanning the recording medium with a laser beam which is modulated in accordance with the signals. This method is advantageous in that recording images can be obtained on real time.
The recording layer of such thermal recording medium is usually made of metals, dyes, or plastics, as described in "Electron, Ion, and Laser Beam Technology", by M. L. Levenc; The Proceedings of the Eleventh Symposidum (1969); "Electronic" (Mar. 18, 1968), P. 50; "The Bell System Technical Journal", by D. Maydan, Vol. 50 (1971), P. 1761; and "Science", by C. O. Carlson, Vol. 154 (1966), P. 1550.
According to these disclosures, some of the metal recording layers are formed by depositing a thin film of Bi, Sn, or In on the substrate. Such recording layers are superior as thermal recording media because they permit the recording of images with good resolution at high contrast.
The substrates for such recording media have conventionally been comprised of glass plate and plastics film. The glass plate is generally used because of its good surface properties. However, a glass plate is not desirable because it can be easily broken during use and storage. On the other hand, plastic films are not fragile, but poorer in the surface properties as compared with glass plates. The poorer surface properties decrease the S/N ratio of reproduced signals in the case of a thermal recording medium in which a recording layer of metal thin film is formed directly on the plastics substrate. In order to overcome this problem, efforts have been made to provide a polymer layer between the recording layer and the plastics substrate so that the recording layer is not adversely affected by the poor surface properties of the plastics substrate. A variety of polymers are available for such purposes. (For example, see Japanese Patent Application (OPI) No. 14504/1980 which corresponds to British Patent 2,026,346.) (The term "OPI" as used herein refers to a "published unexamined" Japanese Patent Application).
The recording medium described above is used by recording the information by irradiating the laser beam onto the recording layer from the substrate side. The recorded information is read by irradiating the laser beam for reading from the substrate side. The substrate for such a system is preferably comprised of acrylic resin such as polymethyl methacrylate and methyl methacrylate copolymer which are transparent to the laser light. The acrylic resin substrate, however, has poor adhesion with respect to the metallic recording layer. A recording medium used as an optical disc memory must be capable of keeping information for more than 10 years. Any decrease of adhesion with time leads to poor reliability. In order to overcome this disadvantage, the present inventors tried to use the undercoating of conventional polymers and the polymer as disclosed in the above-mentioned Japanese Patent Application (OPI) No. 14504/1980. However, it was not possible to obtain improved adhesion or prevent the adhesion from degrading with time. In some cases, the undercoating adversely affected the sensitivity of the recording layer.